An electrophotographic process usually proceeds in sequence as programmed: pressing a copying start key; driving a photoreceptor; charging the photoreceptor; forming a latent image by exposure; developing the image; supplying paper; transferring the image to the paper; fusing the image; cleaning the photoreceptor surface; and removing residual electrical potential from the photoreceptor. Through such an electrophotographic process a copy of an original image is produced.
In recent years, in order to meet a market demand for high quality copies, factors of the electrophotographic process such as the charged potential of the photoreceptor, the lamp voltage of an optical system and the toner density are controlled so as to obtain solid black and half-tone images of a uniform density. Namely, an electrostatic latent image or a toner image is formed on a portion of the photoreceptor surface and the current information about the surface potential at the portion or the density of the toner image is supplied to a control circuit. Control information for producing good images is thus derived from the current information.
More specifically, the charged potential of the photoreceptor and the surface potential after exposure are measured with a surface electrometer. The resulting values are respectively compared with a reference potential. According to the difference an applied voltage of a charger and the lamp voltage of the light source are controlled. In an alternative method, a patch of a uniform density is placed in a portion of a document platen, the toner image of the patch is formed in a portion of the photoreceptor and the density of the toner image is measured with an optical sensor. The ratio of toner to developer is controlled according to the result of the measurement. Such a control is performed before transferring the toner image to copy paper.
With a conventional structure, however, an electrostatic latent image or a toner image of the patch can not always be formed in a predetermined portion on the photoreceptor surface. The reason for this is as follows.
The electrostatic latent image or the toner image of the patch is formed on the photoreceptor surface by an exposure operation or a development operation after exposure. Exposure or development of the image is performed when a predetermined time elapses after a copying start instruction is input through, for example, the copying start key. The position of the photoreceptor at the time the copying operation starts is not determined. As the position of the photoreceptor changes arbitrarily, the electrostatic latent image or the toner image of the patch is formed in various portions of the photoreceptor surface.
Thus, even when a patch of a uniform density is used, there are relatively wide fluctuations in the values obtained from the electrostatic latent image or the toner image of the patch are not uniform due to the configuration (circularity) and mechanical variations of the photoreceptor such as rotational displacement, or variations in the photoconductivity at different portions of the photoreceptor surface. Namely, since the control information for producing good quality images varies every copying operation, copies of optimum image quality can not be obtained constantly.